The modernization of antenna technologies in practice is moving forward in a high pace, which enables the use of more advance antenna setups and techniques in order to increase throughput and robustness in wireless networks such as mobile radio networks. One such technique is to shape the beams, so-called beamforming, of the antenna array. With the use of so called reconfigurable antenna systems, which are already available in practice, beamforming is made possible. The gains that can be achieved with such techniques are promising and seem to be of great importance to achieve the goals of future radio networks.
Recent advancements in antenna technologies include so-called Hierarchical Beamforming, HBF, which is based on dividing antenna array elements into groups to form sub-beamforming arrays [1, 2].
The gains that are promised with advanced beamforming do not come for free as several new problems open up. There are many different ways to implement beamforming using an antenna array, and many of the high-level techniques are known in the academia.
One of the problems with beamforming relates to the errors in the channel estimation and the overhead in feedback reporting (if it is required). The narrower and more fine-tuned beamforming is used, the more sensitive it is to channel estimation errors.
This can introduce very large throughput degradations if the beamforming is based on a highly mismatched channel estimate. In other words, when the transmit power is directed in a specific direction and this direction is not correctly chosen due to some erroneous decision making, the intended direction may not see any signal power where a UE of interest is located and all the beamforming gain will be directed to an incorrect spatial location. It is therefore important to have robust procedures that can adjust the “narrowness” of the beam, i.e. the beam width. A beam form is said to be narrower than another beam form if a sufficiently large percentage of the signal power is directed within a smaller angle from the antenna array compared to the same percentage of signal power of the other beam form. Otherwise, it is said to be wider. Also, in modern radio standards such as LTE (FDD mode of operation), as the granularity of the beams increase so does the simultaneous overhead of the reference signaling. By way of example, to have robust beamforming with high granularity without impractical overhead, the techniques available today are not sufficient.
There is thus a general need to improve beamforming in wireless communication networks.